This invention relates generally to fuel cells. More particularly, it relates to a fuel cell with an electrode/electrolyte interface having mesoscale three-dimensional features that provide an increased reaction surface area-to-volume ratio.
Fuel cells are electrochemical devices that are becoming increasingly popular as power sources because of their high power density and ease of scaling. In addition, in the case of hydrogen-oxygen fuel cells, water is the only emission. Fuel cells convert chemical energy directly into electrical energy, and are therefore much more efficient than, for example, internal combustion engines.
A typical prior art fuel cell 10 is shown in FIG. 1. A solid electrolyte 12, typically a polymer membrane or solid oxide, is sandwiched between a porous anode 14 and cathode 16. A catalyst (not shown) such as platinum is fixed at both electrolyte/electrode interfaces. Adjacent to the electrodes are plates 18 and 20, typically made of graphite, that serve as both flow routers for the gases and current collectors for generated electrons. The separator plates 18 and 20 also provide mechanical support for the electrodes and electrolyte, and may also include channels for discharge of water.
In operation of a hydrogen-oxygen fuel cell, hydrogen enters through an inlet and is routed through channels 22 in the top plate 18 and through the anode 14. Upon contact with the catalyst particles at the anode/electrolyte interface, the hydrogen dissociates into electrons and protons. The protons pass through the electrolyte 12, while the electrons flow through an external circuit via the anode 14 and current collecting plate 18. Oxygen enters the cell through a separate inlet and is routed through channels 24 in the bottom plate 20 and through the cathode 16. At the cathode/electrolyte interface, oxygen combines with the electrons from the external circuit and the protons flowing through the electrolyte 12 to produce water. The channels 22 and 24 can also be used to remove water from the cathode 16. Because each fuel cell 10 can provide only about 1 V, a number of fuel cells are combined in series to provide sufficient power for the intended application.
Current work in fuel cell design aims to provide smaller, lighter, more efficient, and less expensive devices. For example, platinum is very expensive, and so providing alternative catalysts and increased catalytic surface area are desirable. More efficient gas flow routing with lighter current collection plates is also desired. For an explanation of the importance of flow field design, see U.S. Pat. No. 5,686,199, issued to Cavalca et al. In addition, an optimal water level that keeps the polymer membrane hydrated while allowing for efficient removal of water from the cathode catalyst sites is required.
U.S. Pat. No. 5,252,410, issued to Wilkinson et al., discloses a fuel cell with integral reactant flow passages in the electrode layers, thereby eliminating the need for heavier separator plates containing channels for flowing the reactant gases. The resulting fuel cell has a higher power-to-volume ratio than conventional fuel cells having reactant flow passages in the separator plates. Similar advantages are provided by a fuel cell system disclosed in U.S. Pat. No. 5,234,776, issued to Koseki. The fuel cell of Koseki includes ribs formed in the electrodes or electrode chambers. The ribs provide efficient water and reactant gas distribution. While both of these patents provide advantages in reactant distribution and water management, they do not address problems of catalyst or reaction surface area.
U.S. Pat. No. 6,149,810, issued to Gonzalez-Martin et al., provides a proton exchange membrane having internal passages parallel to the membrane surface. When used in a fuel cell, water flows through the membrane to hydrate the membrane directly and thereby enhance proton transfer through the membrane. There is no routing of reactant gases by the membrane. The problems of gas distribution and reaction or catalyst surface area are not addressed.
U.S. Pat. No. 4,272,353, issued to Lawrance et al., discloses polymer electrolyte catalytic electrodes that are formed by roughening the surface of a solid polymer electrolyte and depositing a catalyst on the roughened surface. Compared with conventional electrodes, the electrodes of Lawrance et al. provide superior performance with significantly lower catalyst loading. Roughening is performed by abrading the membrane with, e.g., a silicon carbide sheet. In order to achieve the benefits of the invention, the polymer membrane must be abraded, preferably in two orthogonal directions, and not simply patterned by embossing or stamping.
U.S. Pat. No. 5,480,737, issued to Satake et al., provides a solid oxide electrolyte fuel cell containing a power generation layer including a fuel electrode, a solid oxide electrolyte, and an oxygen electrode. Both faces of the power generation layer contain dimples of specific height, diameter, and pitch. The dimples are designed to increase the reaction area while not hindering gas flow through the electrodes. The dimples are cylindrically shaped; Satake et al. do not recommend rectangular shaped dimples, which would create too large a pressure drop in the reactant gases. Even with cylindrical dimples, the aspect ratio is constrained to particular values to prevent significant pressure drop. The dimples are formed in a regularly repeating pattern that is fabricated by pressing the layer in metal molds before it is fired. A drawback of the dimple array design of Satake et al. is its inability to manage dead zones, areas across the major surface in which mass-displacement flow is insufficient. Very little reaction occurs in dead zones, which typically occur in corners, behind the location at which the inlet flow fans out, and at the location where the exit flow constricts.
U.S. Pat. No. 4,816,036, issued to Kotchick, discloses a solid oxide fuel cell containing fuel and oxidant passageways extending through the core of the fuel cell. A trilayer structure containing the anode, electrolyte, and cathode is corrugated to form the reactant passageways. As a result, the device provides increased power density. However, because of the parallel structure of the passageways, the fuel cell requires large manifolds at the ends of the passageways to direct gas flow. Although the fuel and oxidant flow can be in opposite directions, all of the fuel flow and all of the oxidant flow must be in the same direction.
A similar structure is provided in U.S. Pat. No. 4,761,349, issued to McPheeters et al., which discloses a solid oxide fuel cell having a corrugated monolithic core defining reactant flow channels. As with the device of Kotchick, bulky inlet and outlet housings are required to direct the reactant flow into appropriate channels, and all of the fuel or oxidant channels provide flow in one direction only.
Although the prior art fuel cells provide improvements over conventional fuel cells, further improvements in device efficiency are still desired.
Accordingly, it is a primary object of the present invention to provide a fuel cell with a high reaction surface area-to-volume ratio and therefore a high volumetric power density.
It is a further object of the invention to provide a fuel cell with efficient routing of reactant gases. The routing is provided by the shape of the membrane and not with additional separation plates, resulting in a very lightweight device.
It is an additional object of the invention to provide a fuel cell with efficient management of water by channels integral to the electrolyte.
It is another object of the present invention to provide a fuel cell electrolyte having three-dimensional features that provide structural rigidity to the device, thereby reducing overall device size.
These objects and advantages are attained by a fuel cell having three-dimensional features at one or both electrode/electrolyte interface. The features are in a prescribed pattern that can be created using known and novel micromachining techniques. The features provide increased reaction surface area-to-volume ratio, integral reactant flow channels, and enhanced structural rigidity, thereby allowing for significantly decreased device size.
Specifically, the present invention provides a fuel cell having two electrodes and an electrolyte sheet sandwiched between the electrodes, thereby defining first and second interfaces. One or both interface has three-dimensional features in a prescribed pattern. Preferably, the features are designed to direct a flow of reactants from an inlet region to an outlet region of the fuel cell, such that the inlet and outlet regions each communicate with only a portion of the three-dimensional features. One method for forming the features is selective removal of material, but other suitable methods can be used. The features have a depth-to-width aspect ratio of at least 1:2 and a width of between 5 and 500 xcexcm. The ratio of the surface area of the patterned interface to the projected surface area of the patterned interface is preferably greater than approximately 2.
Preferably, both the first and second interfaces are patterned with first and second prescribed patterns. The two patterns can be complementary such that the thickness of the electrolyte sheet between the two patterns is approximately constant. The two patterns can also be different from each other, with each design dependent on the type of reactant contacting the interface. Each patterned interface can also contain an additional pattern superimposed on the prescribed pattern. The prescribed pattern and the additional pattern have different length scales.
The electrolyte can be a polymer than is used as a proton-exchange membrane, in which case it is shaped by a method such as direct casting, injection molding, embossing, laser machining, laminated layer assembly, selective plasma etching, blow molding, and autoclaving. Alternatively, the electrolyte can be a solid oxide used as an ion-exchange membrane, in which case it is shaped by a method such as chemical vapor deposition, gel casting, powder sintering, or sol-gel processing. The electrode can include a conductive grid or porous conductive material that conformably contacts the membrane and catalyst.
The present invention also provides a method for making a fuel cell, including the steps of providing a substrate and selectively removing predetermined regions of the substrate using a micromachining technique, thereby creating three-dimensional features of width between 5 and 500 xcexcm in the substrate. Suitable selective removal techniques include laser machining and selective plasma etching. In an alternative method, a mold having three-dimensional features of rectangular cross-section is provided, and the mold is filled with an electrolyte precursor. For example, the method can be direct casting, injection molding, embossing, blow molding, autoclaving, chemical vapor deposition, powder sintering, or sol-gel processing. The mold can also be filled with a sacrificial material that is removed after the electrolyte precursor is added to the mold.